This invention relates to the production of castings by the evaporative pattern casting process. In particular, it relates to the coating of the pattern so as to provide adequate strength during the pouring of the molten metal and also provide adequate permeability for the venting of the gases formed by the vaporization of the expendable pattern by the molten metal.
The evaporative pattern casting process is a procedure that has become of importance in the production of castings for several reasons including the freedom of design that it permits. In addition, it requires only a limited outlay of capital and allows the foundry to pour the same pattern in different metals.
The technology for the preparation of the pattern is readily available since the usual material for its construction is polystyrene although other materials such as polymethylmethacrylate have been used for certain applications. A vital step in the process is the coating of the pattern since the coating not only controls the rate at which gases are produced and removed, but also must be strong enough to support the molten metal during the vaporization of the pattern. In the event that the coating does not provide adequate permeability for gases formed by the evaporation of the pattern during the pouring of the molten metal then the surface of the casting can be marred by what is known as the lustrous carbon defect. Normally only one coating of the refractory material is applied to the pattern to furnish the required thickness of refractory coating.
It is, therefore, an object of the present invention to improve the coatings used on patterns in the evaporative pattern coating process.
Another object of the present invention is the coating of the patterns used in the evaporative pattern coating process in a manner that the gases formed during the process can be properly vented thus avoiding the marring of its surface of the casting by the lustrous carbon defect.
Still another object of the present invention is the production of a coating for the patterns used in the evaporative pattern coating process which retain its shape during the time period when the expendable pattern is being vaporized.
Other objects will be apparent from the ensuing description.